Re: gEDA-user: wacky (or not?) solder stencil ideas

[Dave N6NZ <n6nz@xxxxxxxx>]

DJ Delorie wrote:
>> Yesterday while I was at the TechShop experimenting with the laser 
>> cutter
> 
> You suck ;-)

:)

Actually, TechShop is gearing up for franchising.  By next year this 
time, you, too might have a TechShop close by.  Warning: time sink ahead. :)

Yesterday they were checking out the latest new tool.  A CNC hot wire 
foam cutter... that accepts 8 foot by 8 foot by 10 foot blocks of foam. 
Wanna kayak?  Print one.

> 
>> 2) Slap it in the laser cutter, and print the paste layer on it with 
>> just enough laser power to blast away the blue paint.  (The CO2 laser is 
>> the wrong wavelength for cutting metal, it just reflects off.)
> 
> Huh?  I thought CO2 was what they cut metal stencils with.

No, I believe they are YIG laser.  CO2 just reflects off, the wavelength 
is about 10x what you need to cut stainless or other shiny metals.  CO2 
is great for cutting and etching wood and acrylic.  The big market is 
trophy and corporate awards shops.  CO2 does a very spiffy job on balsa 
wood if you are into modeling.  I do a lot of acrylic for robot chassis 
and other widgets.  Shameless plug: www.bowcritter.com

> 
>> Anyway, DJ, I've looked over your web site where you do basically this 
>> with the toner transfer method.  Got any comments?
> 
> The trick to etching paste masks is to etch from both sides to reduce
> undercut.  That means careful alignment of both sides, which is tricky
> with toner transfer (mostly because you can't see through the paper to
> align it).  Otherwise, the mask'd side is undercut by N mils, where N
> is the thickness of the foil.

Does the undercut really make the stencil unusable? Or is it something 
you can live with?  Flipping a sheet of brass over on the laser cutter 
is probably not possible to do with sufficient precision.  I've done 
some tight tolerance positioning by building fixtures, but I'd never be 
able to flip a sheet of brass and put it down withing a couple of mils.

> 
> The best method I've seen so far is to take generic aluminum foil,
> laminate a photoresist on both sides, and expose/develop.  Now, DIP it
> in FeCl - the aluminum etches almost instantly.  Use as-is - don't
> strip the photoresist!  The combination of foil and resists is just the
> right thickness for solder paste stencils.

What kind of photo resist? Got a pointer?

<snip>
> 
>> Topic two: I've been trying to get a process that makes stencils in 3 
>> mil drafting mylar.
> 
> Have you tried plain old 20lb copy paper?

I've tried 65# card stock, mainly because it was handy.  Too thick for a 
stencil, 40# might be about right?  It cuts very cleanly.  Very nice 
looking mask.  Nice and sharp and repeatable. And not tweaky to cut. 
Unfortunately, cutting paper creates a thin scorch area at the edge of 
the cut.  I'm concerned about contamination from carrying ash residue 
into the solder joint.

> 
>> Third wacky idea: TechShop has one of those PCB mils.
> 
> I've seen someone do it with just a CNC drill - one drill hole for
> each pad, leaves a spot of paste.  Won't work for TSSOP, but will for
> 0603 et al.
> 
> For fine-pitch parts, it's almost easier to etch a slot along the pins
> and rely on surface tension to put the solder where it's needed.

Makes sense.

-dave


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